An underwater pipeline is normally assembled on board a laying vessel, from which it is laid directly as it is assembled. The pipeline itself comprises a number of pipes joined to total lengths of hundreds of kilometers. The pipes are of normally 12-meter standard length, and relatively large diameters ranging between 0.2 and 1.5 meters, and each comprise a steel cylinder, and a coating of polymer material to protect the steel cylinder from corrosion and heat loss.
The pipes are joined at on-land installations into multiple-standard-length pipes, as well as on laying vessels, on which standard-length or multiple-standard-length pipes are joined to others, in turn already joined to other pipes to form the pipeline, which is then laid on the sea bed by the laying vessel.
The assembly method comprises a series of assembly jobs performed on an assembly system. Generally speaking, “assembly work” is intended to mean both joining work, such as welding, and auxiliary work, such as non-destructive weld testing and preparation for further joining work.
Underwater pipelines are currently assembled and laid by laying vessels using two methods, each of which is preferable to other depending on the depth of the sea bed.
A first method is to construct the pipeline on a vertical assembly line, and to lay it in a substantially vertical position, so the portion of the pipeline between the vessel and the bed assumes a J shape. This method is particularly suitable for laying underwater pipelines in very deep water.
A second method is to construct the pipeline on a substantially horizontal assembly line, and to lay it using a laying device which, in the work configuration, serves to guide and support the pipeline along a curved path having a first portion above the surface of the sea, and a second portion below the surface. Using this method, the pipeline assumes an S shape between the laying vessel and the sea bed.
In both the above known methods, the pipeline is advanced with respect to the vessel in a “jogging movement”, which includes both fast-forward movements of the pipeline with respect to the vessel, alternating with slow-forward movements, during which the assembly work is carried out; and fast-forward movements of the pipeline with respect to the vessel, alternating with stops, during which the assembly work is carried out.
Regardless of which laying method is used, relative movement between the pipeline and the vessel must always be controlled using one or more gripping devices.
Gripping devices come in different types. A first comprises jaws that are movable selectively to and from the pipeline to grip it cyclically. Some gripping devices of this type are movable along the axis of the pipeline to grip it successively and accompany it as it moves forward.
A gripping device of this type is described in Patent Applications GB 2,364,758, GB 2,370,335 and WO 2006/027189.
Another type of gripping device comprises so-called track or crawler type tensioners, as described in Patent Application WO 03/074413. Track or crawler type tensioners comprise tracks which grip the pipeline and extend parallel to each other and to the pipeline; and each track comprises jaws hinged to one another and which cyclically grip the pipeline.
Grip, i.e. the ability to hold the pipeline firmly with no movement with respect to the jaws, substantially depends on the amount of friction between the jaws and the pipeline, the pressure exerted by the jaws on the pipeline, and the overall contact surface area between the jaws and the pipeline.
There is a limit, however, to the pressure that can be exerted by the jaws on the pipeline without damaging the pipeline at the grip area. As stated, each jaw comprises a friction pad, which is made of polymer material and serves to adapt to any unevenness along the pipeline, and to distribute pressure equally along the whole contact surface area and between the various jaws. Being deformable, the friction pad also prevents damaging the pipeline, particularly its protective polymer coating.
Despite the undisputed effectiveness of known gripping devices, particularly critical operating conditions call for increased grip to prevent relative slide between the pipeline and jaws. For example, some pipelines are exceptionally heavy, by comprising a secondary pipeline lining a main pipeline (pipe in pipe), and also afford a small grip surface area.
Moreover, as opposed to be constant, friction between the friction pad and the pipeline depends on the condition of the mutually contacting surfaces: wet, dry, dirty, clean, greasy, etc.
Another critical operating condition is the depth of the laying bed: a long portion of pipeline suspended between the laying bed and the gripping device exerts severe pull on the gripping device.
Many operating conditions therefore call for increasing the size of the jaws, but even this has its physical limits.
By way of a solution to the problem, friction pads have been proposed, made of polymer material in which aluminium bosses are embedded to improve grip. One example of this is described in U.S. Pat. No. 3,754,474, in which grip is enhanced by including bosses in the polymer material.
This technique is not without its drawbacks, however, such as rapid deterioration of the friction pads caused by including the metal bosses, and the very real risk, in some cases, of damaging the protective coating of the pipeline.